AMD's Radeon HD 6950, the Cayman Pro, can be unlocked into a Cayman XT
- the full 1536 shader/24 SIMD count enabled with a simple BIOS flash. This
unlocking appears to be the source of the rumors of Cayman being 'the next R300'. While
outright market crushing performance was one aspect of the R300/R350's glory,
its unlocking and overclocking was a large part of its reputation.

Unlocking the Cayman Pro is not as simple as the 9500 of
lore, as it requires more than a modified driver set. Like the ATI Radeon 9800SE
to PRO, or the Radeon X800/850PRO to XT, transformation requires a BIOS flash to enable the extra units. Fortunately,
the always helpful W1zzard at TechPowerUp has created a
step-by-step guide and
tool set for everyone. In fact, there is now a utility included in the
script pack that will mod your own saved Radeon HD 6950 BIOS to unlock the extra
stream cores, but preserve the HD 6950 settings such as memory timings and
original clocks speeds. Note that the XT uses a higher core voltage than the PRO
does, meaning you overclocking headroom may be reduced with the 6950 unlocked BIOS vs using a 6970 BIOS, but it will be more compatible.

AMD Dual BIOS

In light of the Cayman Pro to XT unlock, three rumors from the pre-launch days come to mind. These rumors variously specified that 'Cayman is the next R300', the existence of a switch to add more shaders, and that Radeon HD 6950 product allocations
were cut and Radeon HD 6970 numbers boosted. Put together, it seems clear that these tidbits were pointing to core unlocking the HD 6950 via BIOS flash - now much safer to do thanks to Dual BIOS. If the XT cores and clocks are problematic, simply move the switch to setting 2 and reboot. Back in HD 6950 land, you can return the BIOS switch to setting 1 and reload your original HD 6950 bios. It's like an undo button for awesome.

Here's a brief overview of the product specifications:

AMD Radeon HD 6900 Specifications

AMD Radeon HD 6900 Series

The differences between the HD 6950 and HD 6970 are few, but go a little further than just the BIOS. The flagship Cayman XT is specified as 250W max board power, and is equipped with 6-pin
+ 8-pin PCI-E power connectors; the HD 6950 features a 200W max board power, and is fitted with two 6-pin PCI-E power connectors. Running the full 24 SIMD at the higher XT clocks on the
Cayman Pro board might run into power issues. Fortunately this can be solved with PowerTune, as it allows increasing the max board power of the card by up to 20%. Read more about PowerTune
here.

PowerTune Adjusted Performance

Interestingly, flashing to the HD 6970 bios on the HD 6950 board doesn't change the power limits; equipped with two PCI-E 6-pin power connectors, PowerTune appears to manage the PRO board to 200W TDP limit despite the XT core. Additionally, the XT boards are specified with GDDR5 at 5.5Gbps (1375MHz) while the Hynix H5GQ2H24MFA T2C equipped
Pro is rated to 5Gbps (1250MHz). We saw in our initial look at the Cayman XT that it in fact uses R0C 6Gbps ram, and was capable of running at 6Gbps. However, the
Pro is equipped with 5Gbps GDDR5, rated at 1250MHz - the reference HD 6950 clock speed. This does limit the
Pro to XT unlock, as if the ram won't overclock to XT speeds (1375MHz) you'll have problems using a default HD 6970 BIOS.

Flashing the BIOS

To test the Cayman Pro's unlocking possibilities we used
our AMD Radeon HD 6950 engineering sample, and flashed it to the BIOS from our
HD 6970 engineering sample. As the 6950 was an ES we were quite confident that
it would unlock, however your own results may vary and Rage3D is in no way
liable or culpable for any loss, damage, or costs incurred by your own actions
attempting to modify your card at the expense of your warranty. We already knew
that our Cayman Pro configuration would run at higher than XT speeds, thanks to our initial look at overclocking the HD 6950. Hopefully this will still be true once all 24 SIMD are enabled and rocking at 880MHz.

HD 6950 Stock

HD 6950 Unlocked

HD 6950 Unlocked & Overclocked

In an attempt to determine if the unlock operated on the texture units as well as the stream cores, we ran 3DMark Vantage's texture fill test.

3DMark11

3DMark Vantage Texture fill rate test at 1920x1080 resolution with texture quality set to extreme, all other setting on performance. The 'U/L U/C' designation denotes where the card was unlocked to 6970 specifications, and set to 6950 clock speeds (800MHz engine, 1250MHz memory clocks). The Radeon HD 6970 O/C setting is the card overclocked to 925MHz core, 1425MHz RAM, with PowerTune set to +20%.